There is a database called acapellas4u. Google it and voila. You do have to register and to my knowledge, its free. I used to browse it but never registered to use tracks, since none fit my tastes. You might have to "donate".
I just use YouTube and free online YT downloaders. What I like about this is I can search "Prodigy", who is the man in my world, and the sidebar will have all kinds of matching suggestions. Granted the quality is never stellar and I have to EQ the hell out of the vocals, but I'm no engineer and my beats are never EQ'd great anyway.
Lastly, I sometimes get lucky by visiting record label sites who sell digital downloads. I got some Freddie Gibbs and Alchemist tracks that way. 99 cents is nothing.
Good luck!
P.S. This may be bad form, but go to my profile and check out my Soundcloud page. I have remixed joints with both great and crappy quality acapellas that I found using above methods.
Sorry Fade if this is bad form.